N° d’ordre :
THESE
présentée pour obtenir
LE TITRE DE DOCTEUR DE L’INSTITUT NATIONAL POLYTECHNIQUE DE TOULOUSE École doctorale : SCIENCES DES PROCEDES
Spécialité : GENIE DES PROCEDES ET DE L’ENVIRONNEMENT
Par
Somsaluay SUWANPRASOP
Master of Science in Petrochemistry and Polymer Sciences Chulalongkorn University, Thaïlande
I-Aromatisation de n-hexane et d’essence
sur zeolithe ZSM-5
II-Oxydation catalytique en voie humide du
phenol sur charbon actif
Soutenue le 21 avril 2005 devant le jury composé de :
M. S. DAMRONGLERD (Chulalongkorn University, Thaïlande) Président
M. H. DELMAS (ENSIACET, INP Toulouse, France) Directeur de thèse M. F. STUBER (Universitat Rovira I Virgili, Espagne) Rapporteur M. P. CHAIYAVECH (National Petrochemical Co. Ltd., Thaïlande) Rapporteur M. A. PETSOM (Chulalongkorn University, Thaïlande) Membre Mme C. JULCOUR-LEBIGUE (CNRS, Toulouse, France) Membre
TITRE:
I-Aromatisation de n-hexane et d’essence sur zéolithe ZSM-5
II-Oxydation catalytique en voie humide du phenol sur charbon actif
Thèse de Doctorat : Génie des Procédés et de l’Environnement, INP Toulouse, FRANCE, 2005.
Laboratoire de Génie Chimique, UMR CNRS 5503, 5 rue Paulin Talabot, 31109 TOULOUSE
RESUME:
I - L’aromatisation de n-hexane et d’essence brute sur Zéolithe ZSM-5 au
Palladium est étudiée en réacteur tubulaire. Les meilleures conditions ont été obtenues à 400°C, 0,4 mL/min d’alimentation en réactifs, avec une Zéolite ZSM-5 (à 0,5% de Pd). Dans ces conditions les conversions en n-hexane et en essence sont respectivement de 99,7% et 94,3%, avec des sélectivité respectives de 92,3 et 92,6%.
II - L’oxydation catalytique en voie humide du phénol est étudiée en réacteur à
lit fixe de charbon actif commercial à températures et pressions modérées. Les concentrations en sortie, de phénol de DCO et des intermédiaires montrent que la réaction est essentiellement favorisée par la température, la pression d’oxygène et le temps de contact. Au contraire l’hydrodynamique (débit de gaz et mode d’écoulement ascendant ou descendant) ne joue qu’un rôle mineur. Un modèle complet associant la cinétique intrinsèque et les multiples transferts de matière simule bien le comportement du réacteur.
MOT CLES: aromatisation essence
hexane zéolithe
oxydation phénol
TITLE:
I-Aromatisation of n-hexane and natural gasoline over ZSM-5 zeolite II-Wet catalytic oxidation of phenol on fixed bed of active carbon
Ph.D. Thesis: Chemical and Environmental Engineering, INP Toulouse, FRANCE, 2005. Laboratoire de Génie Chimique, UMR CNRS 5503, 5 rue Paulin Talabot, 31109 TOULOUSE
ABSTRACT:
I - The production of aromatic hydrocarbons from n-hexane and natural
gasoline over Pd loaded ZSM-5 zeolite in a tubular reactor was achieved under the suitable conditions at 400 °C, and 0.4 ml/min reactant feeding rate, employing ZSM-5 (0.5% Pd content) as a catalyst. Under these conditions, n-hexane and natural gasoline conversions were found to be 99.7% and 94.3%, respectively (with respective aromatic selectivity of 92.3% and 92.6%).
II - Wet catalytic air oxidation of phenol over a commercial active carbon was
studied in a three phase fixed bed reactor under mild temperature and oxygen partial pressure. Exit phenol concentration, COD, and intermediates were analysed. Oxidation of phenol was significantly improved when increasing operating temperature, oxygen partial pressure, and liquid space time, while up or down flow modes had only marginal effect. A complete model involving intrinsic kinetics and all mass transfer limitations gave convenient reactor simulation.
KEYWORDS: aromatisation natural gasoline
hexane zeolite
oxidation phenol
iv ACKNOWLEDGEMENTS
I would like to express my deepest gratitude to my advisors, Associate Professor Dr. Amorn Petsom and Professor Dr. Henri Delmas, for their valuable
instruction, concern, and encouragement throughout this study. I am grateful to Dr. Carine Julcour-Lebiege for her kind suggestion, instruction, and great help. I would also like to acknowledge Professor Dr. Anne-Marie Wilhelm for her valuable
advice and instruction. I am also thankful to Professor Dr. Frank Stüber for his valuable suggestion and great help. I would also like to acknowledge Professor Dr. Sophon Roengsumran for his kind instruction and valuable advice. I am grateful to Professor Dr. Pramote Chaiyavech for his valuable suggestion and advice. I would also like to acknowledge Professor Dr. Somsak Damronglerd for his valuable advice and suggestion.
I would like to thank the Chairman and Members of the Thesis committee for their valuable suggestion and comment.
I am grateful to the Department of Chemistry and the Institute of Biotechnology and Genetic Engineering (IBGE), Chulalongkorn University for the GC/MS and GC facilities, respectively. I am indebted to the Royal Golden Jubilee Ph.D. Program, The Thailand Research Fund for a student scholarship. I would also like to acknowledge the French Embassy in Thailand for kind support and convenient for the research work in France. My thanks also go to Dr. Prasat Kittakoop for his comments and suggestion on this thesis.
I would like to thank Lahcen, Alain, Lucien, Jean-Louis L., Jean-Louis N., Richard and Marie-Line for their help on the fixed bed set-up and the HPLC at INP. I would also like to thank Mr.Yi Yue, my colleague, for his friendship and kind help for the experiments at INP. I am thankful to Miss Elisabet Agullo, my colleague, for her experiments on the kinetic study of phenol oxidation. My thanks also go to my friend, Dr. Rojrit Rojanathanes, for the discussion on the mechanism of phenol oxidation. I am also thankful to my friends and colleagues for their friendship and encouragement.
I am grateful to my parents and my brother for their love, understanding, and great encouragement throughout this study.
v CONTENTS
PAGE
Abstract in French……… ii
Abstract in English………... iii
Acknowledgement………. iv
Contents………. v
List of Figures………... xiii
List of Tables………. xviii
List of Abbreviations and notations……… xx
PART I: AROMATISATION OF n-HEXANE AND NATURAL GASOLINE OVER ZSM-5 ZEOLITE xxiv CHAPTER I. INTRODUCTION……...……….… 1
II. THEORY AND LITERATURE REVIEW………...………... 3
2.1 Catalysis……….…….. 3 2.1.1 Catalysis activity……….……. 4 2.1.2 Catalysis selectivity………. 5 2.1.3 Catalysis deactivation……….. 6 2.1.3.1 Catalyst poisoning………... 6 2.1.3.2 Fouling………. 6 2.1.3.3 Sintering……….….. 7
2.1.3.4 Loss of catalyst species via the gas phase………... 7
2.2 Classification of catalyst……….……. 7
2.2.1 Comparison of homogeneous catalysis and heterogeneous catalysis………... 8
2.3 Heterogeneous catalysis……….. 9
2.3.1 Individual steps in heterogeneous catalysis………. 9
2.3.2 Promoters………...……….. 10
2.4 Zeolites……… 11
2.4.1 Structure of zeolites………. 11
vi PAGE 2.4.2.1 Shape selectivity……….. 14 2.4.3 Acidity of zeolites……… 14 2.4.4 Metal-doped zeolites……….... 18 2.5 Literature review……….. 19 2.6 Practical application………. 23 2.6.1 Cyclar process……….. 23 2.6.2 RZ-Platforming Process………... 24 2.6.3 Alpha process ………..….... 24
2.6.4 Toray TAC9 Process ………... 25
III. EXPERIMENTAL SECTION………. 26
3.1 Materials and general methods……… 26
3.2 Aromatization reactor……….. 26
3.3 Aromatization procedure………. 27
3.4 Preparation of Pd/ZSM-5 catalyst………... 28
3.4.1 Preparation of 0.2% Pd/ZSM-5 catalyst……….. 28
3.4.2 Preparation of 0, 0.3, and 0.5% Pd/ZSM-5 catalyst…………. 28
3.5 Characterization of Pd/ZSM-5 catalyst……….... 28
3.6 Aromatization of n-hexane……….. 28
3.6.1 Various effects on aromatization of n-hexane………. 28
3.6.1.1 Effect of Pd contents in ZSM-5 zeolite………... 28
3.6.1.2 Effect of reactant feeding rate and reaction temperature………... 28
3.6.2 Regeneration of spent catalyst………. 29
3.6.3 Activity of regenerated catalyst………... 29
3.7 Aromatization of natural gasoline………... 29
3.7.1 Various effects on aromatization of natural gasoline……….. 29
3.7.1.1 Effect of Pd contents in ZSM-5 zeolite………... 29
3.7.1.2 Effect of reactant feeding rate and reaction temperature………... 29
3.7.2 Regeneration of spent catalyst………. 30
vii PAGE
3.8 Characterization of n-hexane and natural gasoline aromatization
products………... 30
3.9 Characterization of n-hexane and natural gasoline……….. 30
IV. RESULTS AND DISCUSSION……….….….…. 31
4.1 Characterization of ZSM-5 zeolite………..…. 31
4.2 Preparation of Pd/ZSM-5 catalyst by ion-exchange method……... 31
4.2.1 Preparation of 0, 0.2, 0.3, and 0.5% Pd/ZSM-5 catalysts…… 32
4.3 Aromatization of n-hexane……….. 33
4.3.1 Various effects on aromatization of n-hexane………. 33
4.3.1.1 Effect of Pd contents in ZSM-5 zeolite……….…... 33
4.3.1.2 Effect of reactant feeding rate and reaction temperature. 34 4.3.1.3 Activity of regenerated catalyst………... 36
4.3.2 Aromatic contents in reaction product and product distributions………. 37
4.4 Aromatization of natural gasoline……….………... 39
4.4.1 Various effects on aromatization of natural gasoline ………. 39
4.4.1.1 Effect of Pd contents in ZSM-5 zeolite………... 39
4.4.1.2 Effect of reactant feeding rate and reaction temperature. 41 4.4.1.3 Activity of regenerated catalyst………... 42
4.4.2 Aromatic contents in reaction product and product distributions……… 43
V. CONCLUSION………... 47
PART II: WET CATALYTIC OXIDATION OF PHENOL ON FIXED BED OF ACTIVE CARBON 48 CHAPTER I. INTRODUCTION……...……… 49
II. THEORY AND LITERATURE REVIEW………...………... 51
2.1 Wastewater treatment………... 51
2.2 Wet air oxidation fundamental……….… 53
2.3 Catalytic wet air oxidation catalysts…………..……….. 53
viii PAGE
2.3.2 Metal oxides……….…… 55
2.3.3 Active carbon……….…….. 55
2.4 Industrial application of Wet Air Oxidation……… 58
2.4.1 NS-LC process………. 59
2.4.2 Osaka Gas process……….….. 59
2.5 Three phase fixed-bed reactors …………..……….…… 60
2.5.1 Introduction……….. 60
2.5.2 Hydrodynamics of cocurrent gas-liquid fixed bed reactor….. 63
2.5.2.1 Flow regimes……….……... 63
2.5.2.1.1 Flow regimes of cocurrent downflow fixed bed reactor………..……… 63
2.5.2.1.2 Flow regimes of cocurrent upflow fixed bed reactor……….. 65
2.5.2.2 Pressure drop……… 66
2.5.2.3 Liquid holdup………... 68
2.5.2.4 Axial dispersion in the gas and liquid phases……….….. 70
2.5.2.5 Catalyst wetting……… 71
2.5.3 Mass transfer……… 73
2.5.3.1 Gas-liquid mass transfer……….….. 74
2.5.3.2 Liquid-solid mass transfer……….…… 75
2.5.4 Heat transfer………. 76
2.5.5 Application of fixed bed reactors to the CWO of phenol…… 77
III. EXPERIMENTAL SECTION: MATERIALS AND METHOD 80 3.1 Materials ……….………..……….. 80
3.2 Characterization of products ………... 80
3.2.1 HPLC analysis …………...………. 81
3.2.2 Determination of chemical oxygen demand (COD) of reaction products …………...……….…… 82
3.3 Catalytic reactors ……… 82
3.3.1 Autoclave reactor ………..……….…. 83
ix PAGE
3.3.1.2 Operating the reactor …………...……….…… 84
3.3.2 Fixed bed reactor ………..………..………. 85
3.3.2.1 Description …………...……….….….. 85
3.3.2.1.1 Principal circuit of reaction ………. 85
3.3.2.1.2 Temperature control circuit ………. 87
3.3.2.1.3 Data acquisition ………….………. 88
3.3.2.2 Procedure for operating continuous oxidation reactor….. 88
3.3.2.2.1 Catalyst packing ………….………. 88
3.3.2.2.2 Operating the reactor ………….……….. 88
3.3.2.2.3 Reactor shutdown ….………….……….. 89
IV. RESULTS AND DISCUSSION: EXPERIMENTAL AND MODELLING ……….……….……… 90
4.1 Phenol adsorption on active carbon………. 90
4.2 Kinetic study on catalytic wet air phenol oxidation on active carbon ………...………... 94
4.2.1 Evaluation of kinetic parameters ……… 94
4.2.1.1 Interpretation of experimental results ……….. 94
4.2.1.2 Modelling of batch reaction and evaluation of intrinsic kinetic parameters ………..……….. 97
4.2.2 Continuous oxidation in fixed bed reactor….……….. 101
4.2.2.1 Operating conditions and flow regimes ………... 101
4.2.2.2 Transient profiles ………..………... 103
4.2.2.3 Activity of catalyst ………..………. 104
4.2.2.4 Influence of operating parameters on phenol conversion 105 4.2.2.4.1 Effect of temperature ………….………. 105
4.2.2.4.2 Effect of oxygen partial pressure ………….……... 107
4.2.2.4.3 Effect of gas inlet velocity ……..………….……... 108
4.2.2.5 Characterisation of reaction products ……….. 110
4.2.2.5.1 Main intermediates ………….………. 110
x PAGE
4.2.2.5.3 Proposed mechanism for oxidative destruction of
phenol over activated carbon in fixed bed reactor... 117
4.2.2.6 Axial temperature and concentration profiles…………... 120
4.2.2.6.1 Axial temperature profiles ………….………. 120
4.2.2.6.2 Axial concentration profiles ………….…………... 121
4.2.2.7 Considerations on scale-up of phenol oxidation over AC 123 4.3 Modelling of continuous CWO …...……… 126
4.3.1 Fixed bed model and numerical solution ……… 126
4.3.1.1 Model equations ……….…….. 126
4.3.1.2 Numerical solution ……….…….. 131
4.3.2 Evaluation of physicochemical properties and fixed bed parameters ……….. 132
4.3.2.1 Physicochemical and thermodynamic properties……….. 132
4.3.2.2 Hydrodynamic, mass, and heat transfers parameters…… 133
4.3.3 Prediction of pilot plant reactor performance ………. 135
4.3.3.1 Axial temperature profiles ………... 135
4.3.3.2 Outlet phenol conversions and axial concentration profiles ………..…………... 135
4.3.3.2.1 Upflow mode ……….………….……… 135
4.3.3.2.1.1 Outlet phenol conversions……….. 136
4.3.3.2.1.2 Axial concentration profiles ……….. 140
4.3.3.2.2 Downflow mode ……….………….………… 143
4.3.3.2.2.1 Outlet phenol conversions……….…….. 144
4.3.3.2.2.2 Axial concentration profiles ………….…….. 147
4.4 Conclusion ……….………. 151
V. CONCLUSION………... 153
REFERENCE ………...……...……… 155
APPENDICES……….…….. 171
Appendix 1-1A XRD spectrum of zeolite ZSM-5……….…… 172
Appendix 1-2A XRD spectrum of fresh Pd/ZSM-5 catalyst……….…… 173
xi PAGE
Appendix 1-4A: Typical GC chromatogram of products from n-hexane
aromatization……… 175 Appendix 1-5A: Typical GC chromatogram of products from natural
gasoline aromatization……….. 176 Appendix 1-6A: GC chromatogram of standard compounds……… 177 Appendix 1-7A: GC chromatogram of commercial mixture of benzene,
toluene, and xylenes……….. 178 Appendix 2-1A: Correlation for the two-phase pressure drop calculation
in cocurrent downflow and upflow fixed bed reactors.. 179 Appendix 2-2A: Correlation for the calculation of liquid retention or
liquid saturation in cocurrent downflow and upflow
fixed bed reactors ………..….……... 183 Appendix 2-3A: Correlation for the calculation of mass transfer
volumetric coefficient and the gas-liquid interfacial area
in cocurrent downflow and upflow fixed bed reactors…. 186 Appendix 2-4A: Properties of Marlotherm S……….… 190 Appendix 2-5A: Thermogravimetric analysis of fresh and aged
activated carbon ……… 192
Appendix 2-6A: Typical HPLC chromatogram of fast analysis for
phenol ……… 193
Appendix 2-7A: Typical HPLC chromatogram of fast analysis for
phenol oxidation products ………. 194 Appendix 2-8A: HPLC chromatogram of full analysis for standard
solution (commercial compounds) including some of
main phenol oxidation intermediates ……… 195 Appendix 2-9A: Typical HPLC chromatogram of full analysis of
phenol oxidation products ………. 196 Appendix 2-10A: Proposed reaction mechanism for phenol CWAO
based on the identified intermediates in the reaction
products ……….……… 197
xii PAGE
Appendix 2-12A: Axial phenol concentration profiles (2)……… 201 Appendix 2-13A: Axial phenol concentration profiles (3)……… 202
xiii LIST OF FIGURES
FIGURE PAGE
1-2-1 Classification of catalysts……….……. 7 1-2-2 Structural unit of sodalite cage in zeolite………...……... 12 1-2-3 Three-dimensional structure of ZSM-5………. 13 1-2-4 Bifuctionality of metal-doped zeolites: isomerization and
hydrogenation……… 18 1-3-1 Reactor for continuous aromatization of n-hexane and natural
gasoline……….. 27 1-4-1 Effect of Pd contents in ZSM-5 zeolite on conversion and
aromatic contents in reaction product of n-hexane.
Conditions: reaction temperature, 400 oC; feeding rate, 0.4
cm3/min ……….……… 34
1-4-2 Effect of the feeding rate on n-hexane conversion and
aromatic contents in reaction product over 0.5% Pd in ZSM-5 zeolite at different temperatures (C and A represent
conversion and aromatic contents in reaction product,
respectively)………... 35 1-4-3 Comparison of activity between fresh and regenerated
catalysts on n-hexane conversion and aromatic contents in reaction product at different temperatures. Conditions: feeding rate, 0.4 cm3/min; catalyst, 0.5% Pd in ZSM-5 zeolite (C and A represent conversion and aromatic contents in
reaction product, respectively)………...……… 37 1-4-4 Effect of Pd contents in ZSM-5 zeolite on conversion and
aromatic contents in reaction product of natural gasoline. Conditions: reaction temperature, 400 oC; feeding rate, 0.4
xiv
FIGURE PAGE
1-4-5 Effect of the feeding rate on natural gasoline conversion and aromatic contents in reaction product over 0.5% Pd in ZSM-5 zeolite at different temperatures (C and A represent
conversion and aromatic contents in reaction product)………. 41 1-4-6 Comparison of activity between fresh and regenerated
catalysts on natural gasoline conversion and aromatic contents in reaction product at different temperatures. Conditions: feeding rate, 0.4 cm3/min; catalyst, 0.5% Pd in ZSM-5 zeolite (C and A represent conversion and aromatic
contents in reaction product, respectively)……… 42 2-2-1 Schematic diagrams of three-phase packed-bed reactors ……. 60 2-2-2 Flow regime boundaries of non-foaming liquid for cocurrent
gas-liquid downflow fixed-bed reactor ………. 64 2-2-3 Flow regime boundaries (air-water system) for cocurrent
gas-liquid upflow and downflow fixed-bed reactor ……… 66 2-2-4 Flow patterns in trickle flow regime for externally completely
and partially wetted particles ……… 72 2-2-5 Concentration profiles for the gaseous reactant A and liquid
reactant B ……….. 74
2-3-1 Schematic diagram of autoclave reactor ………... 84 2-3-2 Schematic diagram of cocurrent gas-liquid fixed bed reactor .. 86 2-4-1 Experimental and modelled breakthrough curves ……… 92 2-4-2 Adsorption isotherms of phenol on Merck AC at room
temperature ………... 93
2-4-3 Time-evolution of phenol concentration in the liquid phase (normalized by initial phenol concentration). T = 150°C,
PO2 = 3.3 bar, Wcat = 9 g, VL0 = 200 ml, ω = 800 rpm ………. 95
2-4-4 Experimental and calculated time-concentration profiles in
the liquid phase ………. 99
xv
FIGURE PAGE
2-4-6 Location of our experimental conditions in the map regimes
of three phase fixed bed reactors for air-water system ………. 103 2-4-7 Transient phenol concentration profiles for cocurrent upflow
and downflow fixed bed reactors. Conditions: oxygen partial pressure 1.2 bar, oil temperature 160°C, gas flow rate 175
Nl/h, and liquid flow rate 0.55 kg/h………... 104 2-4-8 Phenol conversion versus the liquid contact time for
downflow (open symbols) and upflow (filled symbols) at
different oil temperatures ……….. 106 2-4-9 Phenol conversion versus the liquid contact time for
downflow (open symbols) and upflow (filled symbols) at
different oxygen partial pressures ………. 108 2-4-10 Phenol conversion versus gas velocity for downflow (open
symbols) and upflow (filled symbols) at different operating
conditions ……….. 109
2-4-11 Formic acid concentration profile after phenol conversion in
both upflow and downflow fixed bed reactors ………. 112 2-4-12 Acetic acid concentration profile after phenol conversion in
both upflow and downflow fixed bed reactors ………. 112 2-4-13 Malonic acid concentration profile after phenol conversion in
both upflow and downflow fixed bed reactors ………. 113 2-4-14 Oxalic acid concentration profile after phenol conversion in
both upflow and downflow fixed bed reactors ………. 113 2-4-15 1,4-Benzoquinone concentration profile after phenol
conversion in both upflow and downflow fixed bed reactors ... 114 2-4-16 4-Hydroxybenzoic acid concentration profile after phenol
conversion in both upflow and downflow fixed bed reactors ... 114 2-4-17 Comparison of measured COD values and HPLC-based COD
values in both upflow and downflow fixed bed reactors……... 116 2-4-18a Mechanism for phenol WAO according to Devlin and Harris.. 118
xvi
FIGURE PAGE
2-4-18b Proposed reaction pathway for phenol CWAO based on the
identified intermediates in the reaction products ……….. 119 2-4-19 Axial temperature profiles for PO2 = 1.2 bar, Toil = 140 °C,
uG,inlet = 1.1 × 10-2 m/s (FG = 100 Nl/h), and FL = 1 kg/h in
cocurrent upflow and downflow fixed bed reactors …………. 121 2-4-20 Axial concentration profiles for PO2 = 2.0 bar, Toil = 140 °C,
uG,inlet = 1.1 × 10-2 m/s (FG = 100 Nl/h), and FL = 0.5 kg/h in
cocurrent upflow fixed bed reactor ………... 122 2-4-21 Axial concentration profiles for PO2 = 2.0 bar, Tw = 140 °C,
uG,inlet = 1.1 × 10-2 m/s (FG = 100 Nl/h), and FL = 0.5 kg/h in
cocurrent downflow fixed bed reactor ……….. 123 2-4-22 Phenol concentration for downflow (open symbols) and
upflow (filled symbols) oxidation over active carbon at 0.2
MPa of O2 ………. 124
2-4-23 Outlet phenol concentrations: experimental (◊) and corresponding simulations for fully wetted catalyst (Dad=Dad(Stüber)): kGa=kLa (solid line), kGa=5×kLa (long
dotted line), and instantaneous liquid-vapour equilibrium
(short dotted line)………... 138 2-4-24 Axial phenol concentration profiles: experimental (◊) and
corresponding simulations for fully wetted catalyst: kGa = kLa
(solid line), kGa = 5×kLa (long dotted line), and instantaneous
liquid-vapour equilibrium (short dotted line). Dad=Dad(Stüber) 141
2-4-25 Axial phenol concentration profiles: experimental (◊) and corresponding simulations for fully wetted catalyst: kGa = kLa
(solid line), kGa=5×kLa (long dotted line), and instantaneous
liquid-vapour equilibrium (short dotted line) Dad = 4.2 × Dad
xvii
FIGURE PAGE
2-4-26 Outlet phenol concentrations: experimental (◊) and
corresponding simulations for fully wetted catalyst: kGa = kLa
(solid line), kGa = 5×kLa (long dotted line), and instantaneous
liquid-vapour equilibrium (short dotted line)……….... 145 2-4-27 Outlet phenol concentrations: experimental (◊) and
corresponding simulations for partially wetted catalyst: kGa =
kLa (solid line), kGa = 5×kLa (long dotted line), and
instantaneous liquid-vapour equilibrium (short dotted line) … 146 2-4-28 Axial phenol concentration profiles: experimental (◊) and
corresponding simulations for fully wetted catalyst: kGa = kLa
(solid line), kGa = 5×kLa (long dotted line), and instantaneous
liquid-vapour equilibrium (short dotted line) ………... 147 2-4-29 Axial phenol concentration profiles: experimental (◊) and
corresponding simulations for partially wetted catalyst: kGa = kLa (solid line), kGa = 5×kLa (long dotted line), and
xviii LIST OF TABLES
TABLE PAGE
1-2-1 Comparison of homogeneous and heterogeneous catalysts…... 9 1-2-2 Characteristics of important zeolites……….…. 13 1-2-3 Effect of metal ion in faujasite on cumene dealkylation……… 16 1-2-4 Classification of acidic zeolite according to Si/Al ratio………. 17 1-2-5 Transformation of n-hexane on zeolite catalysts at 380 oC….... 18 1-2-6 Aromatization of light naphtha on zeolite catalysts at 480 oC
and 2.5 bar………..……….... 20
1-4-1 Relationship between concentrations of Pd in ZSM-5 zeolite
and various ion-exchanged times…...……….... 32 1-4-2 Comparison of expected values and experimental values of
the Pd contents of the exchanged catalysts……….……... 33 1-4-3 Contact time between reactants (n-hexane and natural
gasoline) and catalyst at different reactant feeding rates……... 36 1-4-4 Product distributions from continuous aromatization of
n-hexane at the optimal conditions compare to the
corresponding reactant compositions and commercial mixture
of benzene, toluene, and xylenes…….………... 38 1-4-5 Sulfur contents in reactants (n-hexane and natural gasoline),
fresh catalysts, and regenerated catalysts………... 42 1-4-6 Product distributions from continuous aromatization of natural
gasoline at the optimal conditions compare to the
corresponding reactant compositions and commercial mixture
of benzene, toluene, and xylenes……… 43 2-2-1 The applications of noble metal catalysts in the CWAO……... 54 2-2-2 Influence of the oxygen partial pressure on the carbon
consumption …..……… 57
2-2-3 Industrial processes of wet air oxidation ………... 58 2-2-4 Some examples of reactions carried out in trickle-bed reactors 61
xix
TABLE PAGE
2-2-5 Upflow versus downflow cocurrent fixed bed reactors ……… 62 2-3-1 Physical properties of Merck activated carbon 2514…………. 80 2-4-1 Parameters of the Freundlich equation and adsorption capacity
at 5 g/l …….………. 91
2-4-2 Optimised values of Freundlich “a” parameter from
experimental breakthrough curve ……….. 93 2-4-3 Operating conditions of the kinetic study in batch reactor …… 94 2-4-4 Amounts of phenol consumed and re-adsorbed during the
experimental series ……… 96
2-4-5 Intrinsic rate constants at different temperatures from batch phenol destruction experiments with: Wcat = 9 g, stirrer speed
of 800 rpm ………. 100
2-4-6 AC bed dimensions and operating conditions for phenol
oxidation in fixed bed reactor……….…… 102 2-4-7 Values of physical properties in the range of operating
conditions .………. 133
2-4-8 Parameter values and correlations used in up- and downflow reactor modelling of pilot plant at: T = 140 ºC, PT = 6 bar,
FG = 100 Nl/h, FL= 0.5 l/h ……… 134
2-4-9 Influence of kLa, kGa and Dad on upflow model conversion:
comparison with reference case and experimental conversion
at Toil = 140 ºC, FL = 0.5 l/h, FG = 100 Nl/h and PO2 = 1.2 bar.. 136
2-4-10 Influence of particle wetting efficiency on the simulated outlet phenol concentration in downflow mode: PO2 = 1.2 bar, Tw =
xx LIST OF ABBREVIATIONS AND NOTATIONS
a reactor section area (m2)
AC activated carbon
BTX benzene, toluene, and xylenes
ca. about
Cj concentration of compound j (mol/m3)
CG,H2O gas phase concentration of water vapour (mol/m3)
C*L,O2 dissolved oxygen concentration at the gas-liquid interface (mol/m3)
cpL liquid heat capacity (J kg-1 K-1)
cpG gas heat capacity (J kg-1 K-1)
CWAO catalytic wet air oxidation Dc column (reactor) diameter (m)
dp catalyst particle diameter (m)
Dj diffusion coefficient of compound j (m2/s)
Djeff effective diffusion coefficient of compound j (m2/s)
Dad axial dispersion coefficient (m2/s) E activation energy (J/mol)
Eq. equation
f external wetting efficiency (f = fd + fs) FBR fixed bed reactor
FG gas flow rate (Nl/h or Nl/s, at STP)
FL liquid flow rate (kg/h or l/h)
G gas mass superficial flow rate (kg m-2 s-1)
GC gas chromatography
H Henry constant
HPLC high-pressure liquid chromatography
hw wall-to-bed heat transfer coefficient (W m-2 K-1)
∆Hv water enthalpy of vaporisation (J/mol)
∆Hi heat of ith reaction (J/mol)
k0 pre-exponential factor of rate constant (m3 s-1 kg-1)
xxi
kGa gas side-liquid volumetric mass transfer coefficient of water vapour (s-1)
kLa gas-liquid side volumetric oxygen mass transfer coefficient (s-1)
(ka)LL,j dynamic-static liquid volumetric mass transfer coefficient of compound j
(s-1)
kLS,j j-compound liquid-solid mass transfer coefficient (m/s)
L liquid mass superficial flow rate (kg m-2 s-1) LHSV liquid hourly space velocity (h-1)
L
m& mass liquid flow rate (kg/s)
MS mass spectrometry
MT/Y metric ton per year
T , G
n& total molar gas flow rate (mol/s)
O H , G 2
n& molar water vapour flow rate (mol/s) PvH2O water vapour pressure (Pa)
PO2 oxygen partial pressure (Pa or bar)
PT total pressure in the reactor (Pa or bar)
qe amount of phenol adsorbed per unit weight of activated carbon (mol g-1)
r particle radial dimension (m) ri ith reaction rate (mol kg-1 s-1)
rp catalyst particle radius (m)
R universal gas constant (8.314 J kg-1 K-1)
Rj total production or destruction rate of compound j (mol kg-1 s-1)
SSR small-scaled fixed bed reactor T temperature (K or °C)
TBR trickle-bed reactor
Tw reactor wall temperature (K or °C)
uL liquid superficial velocity (m/s)
uG gas superficial velocity (m/s)
.
V volumetric flow rate (m
3/s)
VR reactor volume (m3)
VL liquid volume (m3)
xxii
Wcat catalyst load (g or kg)
WHSV weight hourly space velocity (h-1) wt% percentage by weight
x liquid molar fraction XRD X-ray diffraction
XRF X-ray fluorescence
z (or Z) reactor axial dimension (m)
Greek
α order of reaction of oxygen εL liquid hold up
εp particle porosity
φ’ Weisz modulus for pore diffusion based on observed reaction rate ηj effectiveness factor of compound j
ϕ evaporation rate based on reactor length (mol m-1 s-1)
µ viscosity (Pa s)
ρb apparent bed density (kg/m3)
ρL liquid density (kg/m3)
ρG gas density (kg/m3)
ρp catalyst particle density (kg/m3)
σ surface tension (N/m) τ space time (h or min) ω stirrer speed (rpm)
Superscripts
app apparent
dyn dynamic liquid
sta static liquid
Subscripts
xxiii eq at equilibrium G gas H2O water i reaction index j compound index L liquid O2 oxygen p particle Ph phenol S on catalyst surface T total W wall
PART I
AROMATISATION OF n-HEXANE AND NATURAL GASOLINE OVER ZSM-5 ZEOLITE
1 CHAPTER I
INTRODUCTION
The transformation of alkanes into aromatic hydrocarbons is an area of great industrial relevance and also of academic interest for the production of benzene, toluene, xylene (BTX), and naphthalene derivatives. Aromatic hydrocarbons are important feedstock in chemical industries; for examples, benzene is an important feedstock for the production of polystyrene (PS); toluene is used as feedstock for polyurethane (PU) production; p-xylene is employed in poly(ethylene terephthalate) (PET) production process; and naphthalene is a substrate for the production of phthalic anhydride, an intermediate for dyestuff manufacture [1]. In general, naphtha-reforming catalysts (Pt/Al2O3) have been used to convert heavy naphtha (C7–C10) into
aromatic hydrocarbons, but lower alkanes (C2–C6) are hardly transformed over these
catalysts [2]. Chen and co-workers first found the ability of ZSM-5 zeolite to convert light hydrocarbons to BTX; however, these catalysts give poor results for paraffin conversion because of excessive methane and ethane formation with low selectivity to aromatics [3]. Consequently, there have been numerous reports on various ZSM-5-based catalysts for the conversion of light paraffins into aromatics. To improve the low paraffin conversion, various types of activating agents (e.g., gallium, copper, zinc, and platinum) have been added to zeolite. Several systematic studies have been described [4–11]. However, only a few researchers have studied n-hexane aromatization. Popova et al. reported the batch transformation of n-hexane over Cu/ZSM-5 with 82% conversion and 14% aromatic selectivity [12]. Bhattacharya et al. found that the aromatization of n-hexane over H-ZSM-5 zeolite in batch mode
was enhanced by the promoters ZnO and Ga2O3, while Fe2O3 and Cr2O3 decreased its
aromatization activity [13]. Bhattacharya et al. also reported that ZSM-5 gave the highest aromatization activity compared to those of ZSM-22 and EU-1 [14]. Moreover, Rojasova and co-workers studied the role of zinc in Zn/ZSM-5 zeolite in a batch aromatization of n-hexane, of which the conversion of 47.6% with 50.2% aromatic selectivity was obtained [15].
2
Surprisingly, although there have been several studies using Pd/ZSM-5 on alkanes combustion [16–20], none has reported on the ability of Pd/ZSM-5 toward the aromatization, and the present work is the first aromatization reaction upon Pd/ZSM-5.
Aromatization is of industrial interest since it could be employed for the conversion of petrochemical byproduct that contains alkane (e.g., natural gasoline and light naphtha) into other useful products for other chemical industries. The natural gasoline is a natural gas byproduct, which contains mainly saturated aliphatic hydrocarbons, practically free of olefins and also some aromatics [21]. The present work is aimed at developing a continuous process for the production of benzene and naphthalene derivatives from natural gasoline using n-hexane as a model compound and Pd-loaded ZSM-5 as catalyst.
Objectives of this research
The objectives of this research are aimed at (a) preparing ZSM-5 catalysts with various concentrations of Pd-loaded for the production of aromatic hydrocarbons from n-hexane and natural gasoline; (b) determining both qualitative and quantitative analyses of the reaction products; and (c) optimization of reaction conditions in order to obtain desirable products.
Scope of this research
The scope of this research covers the preparation of Pd-loaded ZSM-5 catalysts (0, 0.2, 0.3, and 0.5 wt%). These catalysts were characterized by XRD and XRF techniques. Product distributions, when using n-hexane and natural gasoline as substrates, are qualitatively and quantitatively determined by GC and GC/MS techniques.
3 CHAPTER II
THEORY AND LITERATURE REVIEW
2.1 Catalysis
In the early years of the 19th century, when many important discoveries of chemistry and physics were being made, it was noticed that a number of chemical reactions were affected by trace amount of substances that were not consumed in the reaction. Berzelius introduced the term catalysis in the early of 1836 in order to explain various decomposition and transformation reactions [22,23]. He assumed that catalysts possess special powers that can influence the affinity of chemical substances
[22,23].
The basic concept of a catalyst is that of a substance that in small amount causes a large change in reaction rate. More precise definitions of catalysis have been gradually presented since the understanding of catalysis phenomena has grown. Nevertheless, even today there is no universal agreement on catalyst definitions, the point of view varying depending upon the investigator. A definition, which was given in the term of physical chemistry law, that is still valid today was presented by Ostwald in 1895, stated that “catalyst is a substance that accelerating a chemical reaction without affecting the position of the equilibrium.” While it was formerly known that the catalyst remained unchanged in the course of the reaction, it is now known that the catalyst is involved in chemical bonding with the reactants during the catalytic process. Thus, catalysis is a cyclic process: the reactants are bound to one site of the catalyst, and the products are released from another, regenerating the initial site. In theory, an ideal catalyst would not be consumed, but this is not the case in practice. Owing to competing reactions, the catalyst undergoes chemical changes, and its activity becomes lower (catalyst deactivation). Catalysts also have another important property, apart from accelerating reaction; they can influence the selectivity of chemical reactions. This means that completely different products can be obtained from a given starting material by using different catalyst system. Industrially, this
targeted reaction control is often even more important than the catalytic activity
4 2.1.1 Catalysis activity [23,24,26]
The activity of a catalyst refers to the rate at which it causes the reaction to proceed to chemical equilibrium [24]. Generally, in kinetic treatment, the reaction rate is measured in a temperature and concentration ranges that will be present in the reactor. The reaction rate r is calculated as the rate of change of the amount of substance nA of reactant A with time relative to the reaction volume or the mass of catalyst:
r = (mol Lconverted amount of substance of a reactant -1 h-1 or mol kg-1 h-1) volume or catalyst mass × time
(1-2-1)
Kinetic activities are derived from the fundamental rate laws, for example, that for a simple irreversible reaction A→ P:
= kV f(cA) (1-2-2)
dn
Adt
The temperature dependence of rate constants is given by the Arrhenius equation:
k = k0 e-(E /RT)a (1-2-3)
Ea = activation energy of the reaction k0 = pre-exponential factor
R = gas constant
As Equations (1-2-2) and (1-2-3) show, there are three possibilities for expressing catalyst activity: reaction rate, rate constant k, and activation energy Ea
[23].
Another measure of catalyst activity is the turnover number (TON) or turnover frequency, which is the number of molecules that react per site per unit time. As a basic measure of true catalytic activity, this is a useful concept, but it is limited by the difficulty of determining the true number of active sites. In general, it is easier to do this for metals than for non-metal catalysts since techniques such as selective chemisorption are available to measure the exposed surface area of metals. For acid catalysts, the measurement of site concentration by poisoning or adsorption of bases
5
may lead to erroneously high value since sites may be active for sorption but not for reaction. As with rates of reaction, the turnover number is a function of pressure, temperature, and composition of reactants [24].
Catalysts are often investigated in continuously operated test reactors, in which the conversions attained at constant space velocity are compared [23]. The liquid hourly space velocity (LHSV) is the volumetric flow rate V0 relative to the reactor volume (unpacked) V:
LHSV =
V V0
(h-1) (1-2-4)
The volumetric flow rate may be calculated at the inlet or reactor conditions, or at standard temperature and pressure (STP) and usually based on the volume of entering reactant. The reciprocal of this is contact time or superficial contact time, which has unit of time. In some cases the LHSV is given in term of volumetric feed rate of a liquid, even though it may be vaporized and mixed with other reactants before entering the catalyst bed.
If the volumetric flow rate and reactor volume in Equation (1-2-4) were replaced by mass flow rate and catalyst mass, respectively, it will give the weight hourly space velocity (WHSV) [23,24].
2.1.2 Catalysis selectivity [24]
The selectivity of the reaction usually defined as percentage of reactant that is converted to the desired product. The selectivity usually varies with pressure, temperature, reactant composition, and extent of conversion as well as nature of the catalyst.
Yield is an engineering or industrially used term which refers to the quantity of product formed per quantity of reactant consumed in overall reactor operation. Within this overall operation there may be recycle of various reactants, as after
separation. Yield is frequently reported on a weight basis, so a yield exceeding 100 wt% may be obtained [24].
6 2.1.3 Catalysis deactivation
A catalyst may lose its activity or its selectivity for a wide variety of reasons. The four most common causes of catalyst deactivation are: [23,24]
1. Poisoning of the catalyst. Typical catalyst poisons are H2S, Pb, Hg, and S.
2. Fouling
3. Reduction of active area by sintering or migration 4. Loss of active species via the gas phase
2.1.3.1 Catalyst poisoning
Catalyst poisoning is a chemical deactivation effect, which is happened by forming strong adsorptive bonds between the catalyst poisons and the active catalyst surfaces, thus blocking the active centers. Therefore, even very small amounts of catalyst poisons can influence the adsorption of reactants on the catalyst. The term catalyst poison is usually applied to foreign materials in the reaction system. If the substances that blocked the active centers are the reaction products that slowly diffuse away from the catalyst surface, these substances are referred as inhibitors.
Metal catalysts are highly sensitive to small amount of impurities in reaction medium. Catalytically active metals make their d-orbitals available for adsorption, and this is the key to understanding both their catalytic activity and their sensitivity to poisons. Particularly strong catalyst poisons are the ions of elements group 15 (P, As, Sb, and Bi) and 16 (O, S, Se, and Te). The poisoning activity depends on the presence of electron lone pairs, which have been shown to form bonds with transition metals on chemisorption. If these are involved in bonding to other elements, then the ions are nonpoisons. Catalyst poisoning can be reversible or irreversible, depending on the reaction conditions [23].
2.1.3.2 Fouling
Fouling is a physical blockage of the catalyst active sites, which might be caused by the deposition of fine powder or carbonaceous deposit (coke). In the case of fouling by carbonaceous deposit, the catalyst activity can be restored by burning to remove the coke from the catalyst surface [24].
7 2.1.3.3 Sintering [23,24]
Sintering is an irreversible physical process leading to a reduction of effective catalytic area. It may result from growing of metal crystallites on a support or decreasing of area of non-support catalyst. The rate of sintering increases with increasing temperature, decreasing crystallite size, and increasing contact between crystallite particles. Other factors are the amount and type of impurities on the crystallite surface and the support composition in supported catalysts.
2.1.3.4 Loss of catalyst species via the gas phase
High reaction temperatures in catalytic processes can leads to loss of active components by evaporation. This does occur with compounds that are known to be volatile, but also by reaction of metals to give volatile oxides, chlorides, or carbonyls
[23,24].
2.2 Classification of catalyst
According to the state of aggregation that they incorporate, catalysts can be classified into two groups: heterogeneous catalyst and homogeneous catalyst (Figure 1-2-1). Homogeneous catalysts Heterogeneous catalysts Heterogenized homogeneous catalysts (e.g. biocatalyst and enzymes) Acid/base catalysts Transition metal compounds Bulk catalysts Supported catalysts Catalysts
Figure 1-2-1: Classification of catalysts
By far the most important catalysts used are the heterogeneous catalysts [23]. Heterogeneous catalysis takes place between several phases. Generally, the catalyst is
8
a solid, and the reactants are gases or liquids. In supported catalysts the catalytically active substance is added to a support material that has a large surface area and that is usually porous.
2.2.1 Comparison of homogeneous catalysis and heterogeneous catalysis [22–25]
Generally, in the heterogeneous catalysis, phase boundaries are always present between the catalyst and the reactants, while in the homogeneous catalysis, catalyst, reactants, and products are in the same phase. Homogeneous catalysts have a higher degree of dispersion than heterogeneous catalysts since each individual atom can be catalytically active. In heterogeneous catalysts only the surface atoms are active. Due to high degree of dispersion, homogeneous catalysts exhibit a higher activity per unit mass of metal than heterogeneous catalysts. The major disadvantage of homogeneous transition metal catalysts is the difficulty of separating catalyst from the product. Heterogeneous catalysts are either automatically removed in the process (e.g., gas phase reaction in fixed bed reactor), or they can be separated by simple method such as filtration or centrifugation. In case of homogeneous catalysts, more complicated process such as distillation, liquid-liquid extraction, and ion exchange must be often used. Table 1-2-1 summarizes the advantages and disadvantages of the two classes of catalysts.
9
Table 1-2-1: Comparison of homogeneous and heterogeneous catalysts[23]
Homogeneous Heterogeneous
Effectivity
Active centers all metal atoms only surface atoms
Concentration low high
Selectivity high lower
Diffusion problems practically absent present (mass-transfer controlled reaction) Reaction conditions mild (50−200 oC) severe (often > 250 oC)
Applicability limited wide
Activity loss irreversible reaction with products; poisoning
sintering of the metal crystallites; poisoning Catalyst properties
Structure/stoichiometry defined undefined
Modification possibilities high low
Thermal stability low high
Catalyst separation sometimes laborious (distillation, extraction)
fixed bed: unnecessary suspension: filtering
Catalyst recycling possible easy
Cost of catalyst losses high low
2.3 Heterogeneous catalysis
2.3.1 Individual steps in heterogeneous catalysis [22–24]
Heterogeneously catalyzed reactions are composed of purely chemical and purely physical reaction steps. For the catalysis process to take place, the reactants must be transported to the catalyst surface. Thus, apart from the actual chemical reaction, diffusion, adsorption, and desorption processes are of importance for the progress of the overall reaction. The total process may be divided into the following seven steps, any one of which can be rate determining.
1. Diffusion of the reactants through the boundary layer to the catalyst surface
2. Diffusion of the reactants into the pores (pore diffusion) 3. Adsorption of reactants on the inner surface of the pores
10
4. Chemical reaction on the catalyst surface
5. Desorption of the products from the catalyst surface 6. Diffusion of the products out of the pores
7. Diffusion of the products away from the catalyst through the boundary layer and into the gas phase
Steps 1, 2, 6, and 7 involve no chemical change. Steps 1 and 2 are the physical processes whereby the reactant are brought through the gaseous or liquid phase surrounding the solid catalyst to the active sites on the catalyst’s surface. This is a diffusion process and the phenomenon is called mass transport or mass transfer. Step 6 and 7 is the process for getting products away from the surface. When either of these is slower than the catalytic rate itself, the rate is determined by the rate of arrival of reactants (or removal of products), which is referred as diffusion limitation or mass-transport limitation. Diffusion limitation at the external surface of catalyst particles is recognized by the following characteristics.
1. The rate is proportional to catalyst weight (or to the concentration of active component) and raised to a power less than unity
2. The rate is increased by improving the movement of the gas or liquid with respect to the catalyst
3. The temperature coefficient and the apparent activation energy are low
[22].
2.3.2 Promoters
Promoters are substances that they are not catalytically active, but increase the activity of the catalysts. The function of these substances, which are added in the catalyst in small amounts, has not been elucidated. There are four types of promoters: (a) structural promoters; (b) electronic promoters; (c) textural promoters; and (d) catalyst-poison-resistant promoters [23].
Structural promoters increase the selectivity by influencing the catalyst surface so that the number of possible reactions for the adsorbed molecules decreases and a favored reaction path dominates.
Electronic promoters influence the electronic character of the active phase by dispersing in the phase and therefore chemically binding with the adsorbate.
11
Textural promoters inhibit the growth of catalyst particles to form larger and less active structures during the reaction. They prevent loss of active surface by sintering, thereby increasing the thermal stability of the catalyst.
Catalyst-poison-resistant promoters protect the active phase against poisoning by impurities, either present in the reactant feed or formed during the reactions.
A catalyst may contain one active component and one or more promoters. Since the above four effects tend to overlap in practice, it is sometimes difficult to precisely define the function of a promoter [23].
2.4 Zeolites
2.4.1 Structure of zeolites [23–27]
Zeolites are water containing crystalline aluminosilicates of natural or synthetic origin with highly ordered structures. They consisted of SiO4 and AlO4
-tetrahedra, which are linked through common oxygen atoms to give a three-dimensional network through which long channel run. In the interior of these channels, which are characteristic of zeolite, are located water molecules and mobile alkali metal ions, which can be exchanged with other cations. These compensate for the excess negative charge in the anionic framework resulting from the aluminum content. The interior of the pore system, with its atomic scale dimensions, is the catalytically active surface of the zeolite. The inner pore structure depends on the composition, zeolite type, and the cations.
The general formula of zeolite is
MIMII0.5[(AlO2)x·(SiO2)y·(H2O)z] (1-2-5) Where MI and MII are alkali and alkali earth metals. The indices x and y denote the oxide variables, and z is the number of molecules of water of hydration. The composition is characterized by the Si/Al atomic ratio and the pore size of zeolites.
Zeolites are mainly distinguished according to the geometry of the cavities and channels formed by the rigid framework of SiO4 and AlO4- tetrahedra. The tetrahedra
are the smallest structure units into which zeolite can be divided. Linking these primary building units together leads to 16 possible secondary building blocks (polygons), the interconnection of which produces hollow three-dimensional structures.
12
The entrances to the cavities of the zeolites are formed by 6-, 8-, 10-, and 12-ring apertures (small, medium, and wide-pore zeolite). A series of zeolites is composed of polyhedra (sodalite or β-cage, Figure 1-2-2), composed of 4- and 6-rings, which can be connected in various manners to give the fundamental zeolite structures. The sodalite cage, which consists of 24 tetrahedra, is generally depicted schematically as a polygon, generated by connecting the center of neighboring tetrahedra with a line. Each vertex of this polyhedron represents a silicon or aluminum atom, and the midpoint of each edge, an oxygen atom.
Figure 1-2-2: Structural unit of sodalite cage (β-cage)
Examples of medium-pore zeolites are the pentasils, which belong to the silicon-rich zeolites. Their polyhedra are composed of 5-rings as secondary building units. These so-called 5–1 units are structurally analogous to methylcyclopentane. Linking of the resulting chains give a two-dimensional pore system, one consisting of zig-zag channels of near circular cross-section and another of straight channels of elliptical shape. All the intersections in pentasil are of the same size [25]. The three-dimensional structure of pentasil is represented in Figure 1-2-3a. The 10-membered rings provided access to a network of intersecting pore within the crystal. The pore structure is depicted schematically in Figure 1-2-3b; there is a set of straight, parallel pores intersected by a set of perpendicular zig-zag pores. Many molecules are small enough to penetrate into this intracrystalline pore structure, where they may be catalytically converted [23,26]. An advantage of these zeolites is the uniformed channel structure, in contrast to zeolites A and Y, in which the pore windows provide access to larger cavities. A well-known representative of this class of zeolites is ZSM-5 (from Zeolite Socony Mobil no.5) [23,27].
13
Figure 1-2-3: Three-dimensional structure of ZSM-5 (a) structure formed by stacking of sequences of layers (b) intracrystalline pore structure (After Gates
[26]).
The aluminosilicates structure is ionic, incorporating Si4+, Al3+, and O2-. When some of the Si4+ ions in the SiO4 tetrahedra in this framework are replaced by
Al3+ ions, as in the ZSM-5, an excess negative charge is generated. A compensating source of positive charge must be added, namely cation, in addition to the framework Si4+ and Al3+. These nonframework cations play a central role in determining the catalytic nature of zeolites. The zeolites are ion exchanger. Bringing an aqueous salt solution in contact with the zeolite leads to incorporation of cations from the salt into the zeolite, replacing some of the nonframework cation initially present [25,26].
Table 1-2-2: Characteristics of important zeolites [23]
Type Pore diameter (nm) Pore aperture
Zeolite Y 0.74 12-ring
Pentasil zeolite 0.54 × 0.56 and 0.51 × 0.55 10-ring (ellipsoid)
Zeolite A 0.41 8-ring
14 2.4.2 Catalytic properties of zeolites [23,26]
In 1962, zeolites were introduced by Mobil Oil Corporation as new cracking catalyst in refinery technology. They were characterized by higher activity and selectivity in cracking and hydrocracking. At the end of 1960s, the concept of shape-selective catalysis with zeolite was introduced to petrochemistry (Selectoforming process), and then zeolite becoming the importance topics in catalysis research and applied catalysis [23,26]. There advantages over conventional catalysts can be summarized as follows:
1. Crystalline and precisely defined arrangement of SiO4 and AlO4–
tetrahedra resulted in good reproducibility in production.
2. Shape selectivity: only molecules that are smaller than the pore diameter of the zeolite can undergo the reaction.
3. Controlled incorporation of acid centers in the intracrystalline surface is possible during synthesis.
4. At above 300 oC, pentasil and zeolite Y have acidities comparable to mineral acid.
5. Catalytically active metal ions can be easily applied to the catalyst by ion exchange or impregnation.
6. Zeolite catalysts are thermally stable up to 600 oC, and can be regenerated by burning of carbon deposit [23].
2.4.2.1 Shape selectivity
The shape selectivity of zeolites is based on the interaction of reactants with the well-defined pore system. A distinction is made between these factors, which can overlap: (a) reactant selectivity; (b) product selectivity; and (c) restricted transition state selectivity.
2.4.3 Acidity of zeolites [23,25–27]
Acid strength of zeolite can be varied by numerous preparation methods (ion exchange, partial dealumination, and substitution of the framework of Al and Si atoms). Direct replacement of the alkali metal ions with protons by treatment with mineral acids is only possible in exceptional cases (e.g., mordenite and ZSM-5). The
15
best method is exchanging of the alkali metal ions by NH4+ ions, followed by heating
the resulting ammonium salts at 500 to 600 oC (deammonization; Equation 1-2-6)
Al O Si O O O O Al O Si O O O O Al HO Si O O O O -NH3 +NH3 NH4+ - -H+ O O O O O O (1-2-6)
Infrared investigations have shown that the protons are mainly bound as silanol groups but have a strongly acidic character due to strongly polarizing influence of the coordinative unsaturated ammonium center. Bronsted acid center are generally the catalytically active sites of the H-zeolites. Weak to moderate strong acid sites can be generated in zeolite by ion exchange with multivalent cations. Owing to the polarizing effect of the metal cations, water is dissociatively adsorbed, and the equilibrium of Equation 1-2-7 is established.
[M(H2O)]n+ [M(OH)](n-1)+ + H+ (1-2-7)
The following order of Bronsted acidity is given for cation-exchanged zeolite: H form >> La form > Mg form > Ca form > Sr form > Ba form. The influence of the exchanged ions is considerable, as shown by the example of cumene dealkylation on faujasite (Table 1-2-3). Reasons for the large differences in reactivity are the different charges on the ions, and the decreasing ionic radii from Na+ to H+ and the associated polarizing power of the ions. The incorporation of transition metal ions into zeolites leads to interesting bi-functional catalysts in which metal and acid centers can act simultaneously.
16
Table 1-2-3: Effect of metal ion in faujasite on cumene dealkylation [23]
Cation Relative activity
Na+ 1 Ba2+ 2.5 Sr2+ 20 Ca2+ 50 Mg2+ 100 Ni2+ 1100 La3+ 9000 H+ 8500 SiO2/Al2O3 1
Another major influence on the acidity of zeolites is the Si/Al ratio. The zeolite can be classified into three groups, according to the Si/Al ratio and the associated acid/base properties (Table 1-2-4). Since the ion exchange capacity corresponds to the Al3+ content of the zeolites, those with lower Si/Al ratios have higher concentrations of active centers.
17
Table 1-2-4: Classification of acidic zeolite according to Si/Al ratio [26]
Si/Al ratio Zeolite Acid/base properties
Low (1 to 1.5) A, X relatively low stability of lattice; low stability in acids;
high stability in base; high concentration of acid groups of medium strength Medium (2 to 5) erionite, chabazite, chinoptilolite, mordenite, Y High (ca.10 to ∞) ZSM-5, dealuminated erionite, mordenite, Y
relatively high stability of lattice; high stability in acids;
low stability in base;
low concentration of acid groups of high strength
Zeolites with high concentrations of protons are hydrophilic and have high affinities for small molecules that can enter the pores. Zeolites with low H+ concentrations, such as silicalite, are hydrophobic and can take up organic components from aqueous solution. The boundary lies at a Si/Al ratio of around 10. The stability of the crystal lattice also increases with increasing Si/Al ratio. The decomposition temperatures of zeolites are in the range of 700 to 1300 oC. The highest proton donor strengths are exhibited by zeolites with the lowest concentration of AlO4- tetrahedra such as ZSM-5 and the ultrastable zeolite HY. These are
superacids, which at high temperature (ca. 500 oC) can even protonate alkanes. It was found that the acid strength depends on the number of Al atoms that are adjacent to a silanol group. Since the Al distribution is non-uniform, a wide range of acid strengths results.
18 2.4.4 Metal-doped zeolites [23,25]
Zeolites are especially suitable as support materials for active components such as metals. Suitable metals are effective catalysts toward hydrogenations and oxidations, whereby the shape selectivity of the carrier is retained.
Reaction at the metal
Reaction at the zeolite
Figure 1-2-4: Bifuctionality of metal-doped zeolites: isomerization and hydrogenation
Important factors influencing the reactions of bifunctional catalysts are the location of the metal, the particle size, and the metal-support interaction. The bifuctionality of the metal-doped zeolite catalysts is explained as for the important example of isomerization and hydrogenation. The metal content facilitates the hydrogenation and dehydrogenation steps, while the acid catalyzed isomerization step takes place under the restricted conditions of the zeolite cavities (Figure 1-2-4) [23].
19
Bifunctional catalysts are used in many reactions, including hydrocracking, reforming, and dewaxing process [25]. They usually contain ca. 0.5% Pt, Pd, or Ni.
2.5 Literature review of n-hexane and natural gasoline aromatization over zeolite catalysts
In 1989, Kawata et al. [28] studied the batch aromatization of n-hexane over galloaluminosilicate and gallosilicate. They found that proton forms of galloaluminosilicate and gallosilicate exchanged with ammonium nitrate solution have much higher activity for the aromatization of n-hexane than Ga3+-exchanged HZSM-5 or Ga2O3-supported ZSM-5. They implied that large amounts of gallium
species are uniformly introduced into zeolite crystalline of galloaluminosilicate and gallosilicate, which promote the dehydrogenation of n-hexane to n-hexene. On the other hand, a proton form of gallosilicate exchange with hydrochloric acid showed lower activity for the aromatization of n-hexane, but the activity of the proton form of galloaluminosilicate was increased by a small addition of Ga3+. They suggested that actives gallium species over galloaluminosilicate and gallosilicate are not gallium species in the framework but those outside the framework.
Popova et al. [12] reported the batch transformation of n-hexane over ZSM-5 zeolite with various metals-loaded, which are 2.2% NaZSM-5, 1.0% CuHZSM-5, 1.75% Cu-0.5% NaZSM-5, and HZSM-5. The results are shown in Table 1-2-5.
Table 1-2-5: Transformation of n-hexane over zeolite catalysts at 380 oC
Catalyst Quantity 2.2% NaHZSM-5 1.0% CuHZSM-5 1.75% Cu - 0.5% NaZSM-5 HZSM-5 Conversion, % 6 81 68 88 Cracking products, % 2.4 48 37 62 Aromatics, % - 16.5 7.6 18.5
They also found that the degree of aromatization is related to carbonium ion formation and depends on the acid strength and copper content of the zeolite.
20
Alario and co-workers [29] investigated the aromatization of light naphtha, which composed of 90% C5-alkane, 5.4% C6-alkane, 3.7% C5-cycloalkane, and 0.9%
C6-cycloalkane, using zeolite ZSM-5 that is modified by treatment with a
fluorosilicate solution (e.g., aqueous (NH4)2SiF6) and ZSM-5 zeolites (Si/Al ratio =
27) doped with Ga. The results are presented in Table 1-2-6. From the results, they concluded that modification of ZSM-5 zeolite with both fluorosilicate solution and gallium significantly improved the aromatic products.
Table 1-2-6: Aromatization of light naphtha on zeolite catalysts at 480 oC and 2.5 bar
Selectivity (wt%) Catalyst Conversion (wt%) methane ethane + ethylene propane + propylene butane + butene olefins + C5 + C6 aromatics ZSM-5 93 30 26 14 18 1 11 NH4ZSM-5 78 27 24 13 17 1 18 3.5% GaZSM-5 95 7 17 28 8 1 39 3.45% Ga-NH4ZSM-5 86 6 16 28 7 1 42
Bhattacharya et al. [14] reported the batch aromatization of n-hexane by three-medium pore zeolites with different pore geometry, which are ZSM-5, ZSM-22, and EU-1 zeolites. The conversion of n-hexane over ZSM-5 is nearly 100% at all temperatures in the range of 450 to 540 oC. In the same range of temperatures, there is between 60 to 90% conversion in the case of ZSM-22 zeolite and between 43 to 51% in the case of EU-1 zeolite. The lower conversions recorded over ZSM-22 and EU-1 zeolites suggested that the reaction is probably constrained by diffusion limitations in the case of the above two zeolites that have small pore apertures (0.55 × 0.45 nm for ZSM-22 zeolite and 0.58 × 0.41 nm for EU-1 zeolite) than kinetic diameter of n-hexane (0.47 nm).
Smirniotis and co-workers [30] investigated the performance of the L, β, and USY zeolites supported platinum and of composites of Pt/BaKL with either Pt/β, or Pt/USY zeolite on the reforming reaction of n-hexane, methylcyclopentane, methylcyclohexane, and their mixtures. They found that the reactions of different mixtures of the above three hydrocarbons over the composite resulted in increased selectivities for C7+ aromatics compared to those calculated as molar averages by
